1. Field of the Invention
This invention relates to an electromagnetic-wave detector, and more particularly, to an electromagnetic-wave detector sensitive to X-rays to be used for an X-ray apparatus for medical diagnosis, an X-ray transmission testing apparatus for metallic materials and the like, an X-ray defect detection apparatus, an X-ray baggage inspection apparatus used at an airport or the like, an X-ray diffraction apparatus, an X-ray analysis apparatus, an X-ray thickness measuring apparatus, an X-ray stress measuring apparatus or the like.
2. Description of the Related Art
Recently, various applicational apparatuses utilizing an electromagnetic wave, particularly, X-rays, are being used. For example, medical X-ray apparatuses (X-ray chest-imaging apparatuses, X-ray stomach-imaging apparatuses and the like), X-ray defect detecting apparatuses for metallic materials, X-ray baggage inspection apparatuses used at air-ports or the like, X-ray diffraction apparatuses, X-ray analysis apparatuses, and the like are widely used utliizing the characteristics of transmission, diffraction, absorption, scattering or the like of X-rays.
FIG. 1(a) illustrates the configuration of a conventional X-ray apparatus for medical use. In FIG. 1(a), there are shown an X-ray generating tube 901, an object 902 to be photographed, a phosphor 903 for converting X-rays into. visible light, and an X-ray film 904. This is an apparatus used when photographing still pictures, such as an X-ray chest imaging apparatus or the like.
FIG. 1(b) illustrates the configuration of another X-ray apparatus for taking moving pictures, such as an X-ray stomach-imaging apparatus or the like. In FIG. 1(b), there are shown an X-ray generating tube 901, an object 902 to be photographed, a phosphor 903 for converting X-rays into visible light, an optical system 905 for focusing visible light onto a sensor, and a two-dimensional photosensor 906 using an image pickup device, such as a CCD (charge-coupled device) or the like. A method of taking images by installing a television camera at the position of the two-dimensional photosensor has been practically utilized. In order to detect X-rays in the above-described apparatuses, an X-ray image is obtained by exposing a silver-halide film to X-rays, or by exposing a structure having phosphor layers in contact with both surfaces of a silver-halide film to X-rays in order to increase the sensibility due to fluorescence emitted from the phosphor layers exposed to the X-rays. In another approach, An X-ray image is obtained by converting X-rays into visible light by projecting the X-rays onto a phosphor, and focusing the visible light onto a photosensor, such as a CCD, or onto a television camera using an optical system.
The above-described method of using a silver-halide film has the advantages that a high-definition image is obtained, and that since portions irradiated by X-rays are directly photographed by the film with unit magnification, it is only necessary to set a film cassette at a side of the object to be photographed opposite to the X-ray source, and therefore the structure of the system is simplified. In this method, however, an X-ray image cannot be obtained in real time, and moving pictures cannot be obtained. Furthermore, it is difficult to utilize the obtained image by performing image processing, and therefore the range of application is limited.
On the other hand, the method of using a photosensor has the advantages that an image can be obtained in real time, moving images can also be obtained, and various kinds of information can be obtained by performing image processing. However, in the conventional method of using a photosensor or a television camera, in order to obtain the image of a large object at a time, a particular reduction optical system is required, thereby increasing the size of the entire system and the production cost. As a result, only large hospitals can use such diagnostic inspection apparatuses. Furthermore, since X-rays are converted into visible light using a phosphor, the efficiency of utilization of X-rays is inferior, and the obtained image is dark because the illuminance of the visible light is low. It is desired to increase the efficiency in locations where only weak X-rays can be used, such as actual medical fields.
Some methods for detecting X-rays without using a phosphor have been proposed. For example, at the 1985 Spring Meeting of the Japan Society of Applied Physics, an attempt to directly detect X-rays by forming a p-i-n-type sensor or a photoconductive sensor using hydrogenated amorphous silicon has been announced. However, since hydrogenated amorphous silicon has a low sensitivity to X-rays, it is difficult to practically use this material by itself. In practice, the sensitivity is increased substantially by two or three digits (orders of magnitude) by providing a phosphor on the sensor (see Extended Abstracts p. 29a-U-7).
Y. Naruse et al. have made an attempt to absorb X-rays in a metal layer, guide photoelectrons generated in the metal layer to a hydrogenated amorphous silicon layer, and detect the photoelectrons (Int. Conf. on Solid State Sensor & Actuators, Tokyo, p. 262, 1987). This attempt is excellent in increasing the sensitivity for X-rays utilizing the photoelectric effect. However, as will be described later, the dark current is large in this structure, thereby providing a big problem when high sensitivity is required (for example, in medical apparatuses where the dose rate of X-rays is small).
As described above, various attempts have been made in order to provide higher sensitivity. However, the above-described problems have not yet been completely solved. Since the above-described problems are common not only for medical apparatuses but also for all electromagnetic-wave detection apparatuses represented by X-ray apparatuses, it has been desired to completely solve these problems.